1. Field of the Invention
This invention relates to a press molding method for an optical element designed to form a highly accurate optical element such as an aspherical lens by press molding.
2. Related Background Art
Recently there has been proposed a manufacturing method for molding a glass lens used in an optical apparatus or the like at one shot without the intermediary polishing step. In this method, as disclosed, for example, in Japanese Patent Application Laid-Open No. 1-87524, high accuracy is achieved by adopting a molding method whereby a glass blank in its fully softened state is first press-molded by a first pressing pressure, whereafter temperature is dropped, and then a glass lens is press-molded by a second pressing pressure.
On the other hand, as described in Japanese Patent Application Laid-Open No. 2-38329, it is also conceived to bring the molding surfaces of upper and lower mold members into contact with a glass gnarl, heating said glass gnarl from around said the mold members by means such as induction heating, and thereafter press-molding it to thereby obtain a uniform temperature distribution of the glass gnarl and avoid the creation of a bad product by a non-uniform temperature distribution, thus accomplishing molding of good yield.
However, in both of the above-described examples of the prior art, a predetermined pressure is applied in the process of deforming the glass blank in press molding. Accordingly, for example, where a molding mold and a glass blank are heated discretely from each other and the glass blank is put into the molding mold immediately before press molding, or where the glass blank put into the molding mold is kept hot or heated by only the heat conduction from the molding mold, if the glass blank is press-molded immediately after being put into the mold, there is a danger of the glass blank being subjected to pressing pressure before the temperature of the central portion of the glass blank is increased to the temperature of the molding mold, and being cracked by thermal stress strain or the molding mold being deformed.
Also, where the glass blank is press-molded at a predetermined time after it is put into the molding mold, not only molding that extends, but particularly a thick glass blank has a considerable difference between the temperature of the surface thereof and the temperature of the central portion thereof, and the difference in the temperature distribution thereof gives rise to the problem as noted above.